Bathrooms which include showers are prone to the fogging of mirrors. Some large bathrooms with air exhaust means do not fog up. Most smaller bathrooms with or without air exhaust means fog up as a result of the warm and humid air created by the shower activity causing moisture condensation on the cooler mirror surface.
The problem of defogging bathroom mirrors by electrical heating of the mirror has a long history. Spencer (U.S. Pat. No. 4,665,304) gives an excellent review of the patent literature and states that "none of these proposals has met with commercial success". We note that this observation still applies. He also specifies conditions of success with which we wholly agree, namely:
(a). Utilize widely available mirror glass. PA1 (b). Compatibility with conventional mirror installations. PA1 (c). Compliance with electrical safety codes. PA1 (d). Economy of manufacture. PA1 1. An extremely low-cost bathroom mirror heater. PA1 2. A low operating cost bathroom mirror heater. PA1 3. A super-safe bathroom mirror heater. PA1 4. A readily newly installed or retrofitted heater.
To (d) we would add: economy of operation.
Spencer cites 120 watts per square meter as the required heating to keep the mirror clear but calls for 200 watts per square meter for initial heating. Van Laethem et al. (U.S. Pat. No. 3,790,748) call for 250 watts per square meter as the heat required to prevent misting.
Chang (U.S. Pat. No. 4,060,712) claims that the surface temperature of the mirror needs to be 7-10 degrees Celsius above ambient whereas Prosser (U.S. Pat. No. 4,956,542) says that 4 degrees Fahrenheit above ambient is the temperature differential required to prevent fogging.
Sebel and Rhea (U.S. Pat. No. 3,887,788) teach the use of a printed circuit board to reduce the cost of the heater, act as a heat barrier, and reduce the danger of electrical shock.